1. Field of the Invention
The instant disclosure relates to a calibration method and a calibration circuit; in particular, to a method and a circuit that calibrate mismatches between a first signal path and a second signal path of a transmitter.
2. Description of Related Art
How to compensate mismatches between an in-phase signal path and a quadrature signal path of a transmitter is rather important with regard to transmitters used in the current wireless communication field.
There are mainly two reasons why there will be mismatches between the in-phase signal path and the quadrature signal path of a transmitter. One is that, it is hard to have a perfect 90 degrees phase difference between the signal waves that are generated by a local oscillator and input to an in-phase signal path and a quadrature signal path respectively. In other aspect, the amplitudes of the signal waves that are generated by a local oscillator and input to an in-phase signal path and a quadrature signal path respectively are not equal. A phase difference and/or an amplitude difference mentioned in the above reasons lead to mismatches between the in-phase signal path and the quadrature signal path. Even though the phase difference and/or the amplitude difference are minor, they still bring an obvious interference affecting the whole communication system, which causes an abnormal quadrature demodulation and results in a loss of the transmitted data and an increase of the bit error rate (BER).
Besides, even though the phase difference and amplitude difference can be calibrated, when the frequency of the input baseband signal varies or the baseband signal is inputted to a communication system having a wider frequency band, there may still be mismatches between the in-phase signal path and the quadrature signal path of the transmitter due to the frequency dependency, because mismatches between local oscillators and between mixers may not be related to the frequency of the baseband signal. The above mismatches due to the frequency dependency are caused by differences between electrical characteristics of the corresponding circuit elements in the in-phase signal path and in the quadrature signal path, such as differences between electrical characteristics of the filter in the in-phase signal path and the filter in the quadrature signal path. In other words, the mismatches between the in-phase signal path and the quadrature signal path of the transmitter may be generated because of certain defects in the electric element manufacturing process, which makes the electrical characteristics of the corresponding electric elements in the in-phase signal path and in the quadrature signal path not match perfectly, further making the signals passing through the in-phase signal path and the quadrature signal path different and generating an unexpected image interference. Accordingly, how to compensate mismatches between the in-phase signal path and the quadrature signal path of a transmitter resulted from the frequency dependency is still a problem to be solved.